Sizing the Page File

Now that you've decided where to locate the page file, the next step is deciding how much space should be dedicated to making it functional without being excessive. In the previous section I said that XP makes the default paging file approximately 1.5 times the amount of installed RAM. I'm not sure where that 1.5 amount was derived, but one popular theory holds it's the combined number of dates the decision-makers have had in the preceding 24 months. That's a different article. In truth, the 1.5 factor that XP uses is just fine for systems with RAM in the range of 128 - 384 MB installed. In systems with 512MB plus of installed RAM it's time to take a closer look at modifying the page file defaults. To make an informed decision it's necessary to understand your individual machine and the loads it frequently encounters.  Good information can be obtained from the Windows Task Manager. To open Task Manager:

Right click the Taskbar, then click Task Manager
Click the Performance tab until it appears as shown below.

When the physical memory (RAM) is used up, XP looks to the paging file for help by paging out its excess requirements. The timing of when this occurs is based on your hardware and the load the computer is experiencing. A sure sign that your computer is under excessive load and making use of the page file is when you experience 'disk thrashing', or a continual writing of data to and from the hard drive. The information that is most useful relating to page files is found in the Physical Memory (K), Commit Charge (K), and Kernel Memory (K) sections. The Totals section is more useful to developers and except for the Processes totals won't enter into this discussion. Read through the explanations below and then we'll see how they apply to the page file.

Note: There is a lot of disagreement among Windows XP experts (and I am by NO means an expert) about what the Totals, Physical Memory (K), Commit Charge (K), and Kernel Memory (K) sections in Task Manager actually illustrate. The sections below are strictly the opinions I've formed from reading many different articles about the paging file. My opinions and suppositions may not be accurate, so use the information as a tool and jumping off point in forming your own opinions about what the sections really illustrate. As always, I welcome feedback on anything in The Elder Geek. Help me to make this an accurate source of information for XP users. - The Elder

Commit Charge (K) - The values displayed in this section are in kilobytes (K/KB). To convert to megabytes (MB), divide by 1024.

• Total: The total amount of physical (RAM) and virtual (Page File) memory currently being used by the operating system and all open applications. This value will increase as applications and files are opened and decrease when they are closed. Commit Charge is also displayed in the lower right side of Task Manager. Dividing the Total (183692) by conversion factor of (1024) yields 179 MB, or the same amount shown lower right by Commit Charge: 179M/1878M.

• Limit: The total amount of physical (RAM) and virtual (Page File) memory that is currently available to the operating system and applications. There are two ways to change this value; install additional RAM or increase the size of the page file. Dividing the Limit (1923908) by conversion factor of (1024) yields 1878 MB, or the same amount shown lower right by Commit Charge: 179M/1878M

• Peak: A combined measure of the physical (RAM) and virtual (Page File) memory that has been used during the current windows session. If the Peak value approaches the Limit value it's a good indication the system needs more memory.

Physical Memory (K) - The values displayed in this section are in kilobytes (K/KB). To convert to megabytes (MB), divide by 1024.

• Total: The total amount of physical (RAM) memory installed in the computer. Dividing the Total (785908) by conversion factor of (1024) yields 768 MB, or the same amount verifiable by right clicking on My Computer then clicking Properties to open the System Properties sheet.

• Available: The total amount of physical (RAM) memory that XP will allow programs to use before it switches to virtual (Page File) memory. By default, XP reserves a quantity of the physical memory for additional demands rather than depleting it totally before accessing the page file. Dividing the Total (520136) by conversion factor of (1024) yields 508 MB. Subtracting the 508 MB Available from the 768 MB Total shows XP has reserved 260MB at this particular point in time.

• System Cache: The amount of physical (RAM) memory that is currently being used to store recently accessed programs and data. By default, Windows XP uses as much physical (RAM) memory as possible to cache programs and data. Dividing the Total (550836) by conversion factor of (1024) yields 537 MB available to the system cache. This one setting alone demonstrates the value of installing extra physical memory. XP automatically releases the system cache memory when it's needed by the operating system or other programs.

Kernel Memory (K): The kernel manages the basic operations of the operating system so it's no surprise that it demands memory to accomplish its function. The values displayed in this section are in kilobytes (K/KB). To convert to megabytes (MB), divide by 1024.

• Total: The total amount of physical (RAM) memory that XP uses for core component operations, including drivers for installed devices. Dividing the Total (64372) by conversion factor of (1024) yields 63MB, or approximately 8% of the installed total. On the example machine I'm using for this discussion, the 63MB is a small portion of the 768 MB total installed physical (RAM) memory. However, on a machine that only has 128MB of RAM, the percentage soars to 49% of the installed total. It's easy to see why the recommended minimum RAM for XP is 128MB and 64MB is the absolute minimum supported. Less than 64MB would not be able to support the core components.

• Paged: The total mount of physical (RAM) memory that XP core components are using that are currently mapped to virtual (Page File) memory. Dividing the Total (48144) by conversion factor of (1024) yields 47MB.

• Non-Paged: The total mount of physical (RAM) memory that XP core components are using that cannot be mapped to virtual (Page File) memory. Dividing the Total (16228) by conversion factor of (1024) yields 16MB. Adding the total of Paged (47MB) and Non-Paged (16MB) yields 63MB, or the total Kernel Memory required for core and driver operations in Windows XP.

So we have all these numbers and percentages, but what useful conclusions can be drawn from them, or are they just a bunch of numbers from people with too much time on their hands? Fortunately, they can provide some valuable insight into a machines performance and that's basically why the paging file exists; to increase performance. A few of the possible conclusions are listed below.

• If the Total Commit Charge exceeds the Total Physical Memory, XP has to use the virtual memory page file that is substantially slower than physical memory, suffering performance degradation as a result. One excellent reason to load the system with RAM.

• Compare the Commit Charge Peak to the Physical Memory Total. If the Commit Charge peak is higher than the Physical Memory Total, not only is the page file being used, but the System Cache is not being used. Keeping the installed RAM well above the average Commit Charge peak allows XP to use the extra RAM for System Cache.

• Keeping the Commit Charge Peak running well below the Total Physical Memory allows you to decrease the size of the page file, reclaiming hard drive real estate that can be used for storage rather than slow virtual memory.

Basically, what all this is saying is put your money into RAM and minimize the size of the page file. The performance will be faster, the system cache will be increased, and the hard drive won't be wasting space on virtual memory that can barely get out of its own way. Unfortunately, what's ideal is not always possible, so fire up Task manager and open a good representation of the programs you normally use. I like to double up on everything I normally have open and running just to be on the safe side. Check the Commit Charge Total (CCT), Commit Charge Peak (CCP), and the Physical Memory Total (PMT). If the CCT seldom gets close to the PMT, setting the page file equal to the amount of installed RAM with no room for expansion might be reasonable. Machines with installed RAM in the range of 512MB would be likely candidates.

Once you enter the range of 768 MB to 1 gigabyte (GB) of installed RAM, a page file of 1.5% times the installed RAM is more than likely going to be a waste of hard disk space. The exceptions might be if you deal heavily with CAD, database, and music or image editing programs that tax system resources far beyond the average. Check the CCP mentioned above to see if you fall into this category, but it's likely a system with this amount of RAM can benefit from having the page file set to a 2MB minimum lower limit and a upper limit equal to the amount of installed RAM, just in case the CCT surpasses the PMT.

Page File Articles Series

[ Paging File ] [Locating the Page File ] [ Sizing the Page File ] [ Physically Setting Page File Size ]
 [ Page Files and Fragmentation ] [ Defragmenting the Page File ] [ Paging File Performance Monitoring ]